A telecommunication system consists of various geographically separated nodes exchanging signals. For example, a cellular telephone system includes towers each housing a base station that transmits and receives RF signals to and from cellular telephone transceivers within the service area of the base station. Signals transmitted over a radio link are attenuated due to distance and such factors as propagation loss and multipath fading. Since the strength of the signal is attenuated during transmission between nodes, signals typically transmitted with significant power, using circuit elements known as power amplifiers (PAs).
Cellular telephone systems employing 2.5G and 3G use sophisticated, non-constant envelop modulation techniques. Examples of such modulation techniques are wide-band code division multiple access (WCDMA), orthogonal frequency division multiplexing (OFDM), multicarrier Global System for Mobile Communications (GSM), and Enhanced Data Rates for GSM Evolution (EDGE). Since the data is encoded under these modulation schemes by amplitude and phase, to achieve highest signal integrity, the output signal of a base station transmitter must be highly linear over a wide dynamic range. The linearity of the PAs is even more important as the cellular phone systems encode higher data rates as more sophisticated systems (e.g., 4G systems) are deployed. Therefore, an ideal PA is expected to pass an input signal through to the output undistorted, with a user-tunable gain and a negligible or minimum delay, and independent of the output impedance of the input signal source.
A real PA, however, is not ideal over its entire operating range. The deviation from linear input-output relationship in a real PA may result in unwanted amplitude variations of the output signal and which interferes with signals in other radio channels (e.g., by injecting signals of unwanted harmonics at adjacent radio frequency ranges). A cellular wireless communication system, for example, has a need for a highly linear PA to provide an output signal that achieves a high adjacent channel leakage ratio (ACLR) and a low error vector magnitude (EVM).
To suppress unwanted PA nonlinearity, a predistortion circuit is provided to model the PA's gain and phase characteristics. The predistion circuit provides a pre-distortion signal, which is then combined with the PA's input signal at the input of the PA. Correctly modeled, the output signal of the PA from the combined signal is that of an overall system that is more linear, as compared to the same system without the contribution of the predistortion signal. Thus, purposely introduced predistortion into the input signal of the PA corrects non-linearity in the output signal of the PA. One example of such a system is provided in U.S. Pat. No. 7,844,014 entitled “Pre-Distortion Apparatus” that issued on Nov. 30, 2010, incorporated herein by reference.
FIG. 1 is a functional block diagram for system 100, which is a linearized mixed-signal power amplifier in a base station. As shown in FIG. 1, RF signal source 110 includes digital modulator 105 for modulating baseband data 100 onto a carrier signal (not shown) provided by local oscillator (LO) 108. Digital modulator 105 may be, for example, a digital-to-RF quadrature modulator. The RF signal from digital modulator 105 is transmitted from RF signal source 110 over forward RF path 120 to PA module 130A using suitable means (e.g., over a coaxial cable). Many applications require separating RF signal source 110 from PA module 130. PA module 130 may include RF power amplifier linearizer (RFPAL) 135, which is coupled to RF path 120 via RF couplers 131 and 132. RFPAL 135 performs adaptive analog predistortion of the RF signal on RF path 120, prior to amplification by PA 138. The output signal of PA 138 is fed back using RF coupler 133 to RFPAL 135 to provide adaptive control.
FIG. 2 is a functional block diagram of RFPAL 135. RFPAL 135 has three subsystems, as follows: an RF predistortion block (RFPD) 210, an RF signal analyzer (RFSA) 220 and a micro-controller 230. RFPD 210 may be implemented, for example, by a RFPD disclosed in co-pending U.S. patent application Ser. No. 12/939,067 entitled “Analog Signal Processor for Nonlinear Predistortion of Radio-Frequency Signals” that names as inventor Qian Yu and others and was filed on Nov. 3, 2010, incorporated herein by reference. Similarly, RFSA 220 may be implemented, for example, by an RFSA disclosed in co-pending U.S. patent application Ser. No. 12/340,032 entitled “Integrated Signal Analyzer for Adaptive Control of Mixed-Signal Integrated Circuits” that names as inventor Qian Yu and others and was filed on Dec. 19, 2008, incorporated herein by reference.
RFSA 220 derives from signals RF input 201 and RF feedback 203 a complex-valued error signal that represents the waveform distortion. In addition, RFSA 220 also provides a real-time estimate of the power spectral density (PSD) of the error signal. The PSD and the complex-valued error signal are provided as data signal 222 to micro-controller 230, which provides coefficient vector 232 to RFPD 210. Coefficient vector 232 allows RFPD 210 to perform adaptive nonlinear analog signal processing on RF input signal 201. Because of the limitations inherent in analog circuits, however, it is difficult to realize all the desired improvements by way of signal processing techniques using only analog circuits. What is needed is a system that provides improved linearization of power amplifier in a base station without losing the advantages of the analog signal processing performed by RFPAL 135.